bug#70415: 30.0.50; [Cygwin] `lock-file' creates a symlink with funny name

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 17 Apr 2024 08:58:42 +0900
> From: Katsumi Yamaoka <yamaoka@jpl.org>
> Cc: 70415@debbugs.gnu.org
> 
> On Tue, 16 Apr 2024 16:32:17 +0300, Eli Zaretskii wrote:
> >> .#FILE -> username@hostname.1234:-11644473599
> >> For that case `(unlock-file "FILE")' issues this warning:
> >> Warning (unlock-file): Unlocking file: Invalid argument, FILE, ignored
> 
> > The part after the colon is the system boot time, and is used on all
> > Posix hosts.
> 
> Oh, I had never seen it until recently.  Thanks.

The question is: why it started appearing only recently?  The code in
filelock.c hasn't changed since last December, if not earlier, so if
this started happening recently, it could only be because Gnulib's
get_boot_time or its Cygwin implementation changed, such that it was
previously returning zero whereas now it returns non-zero (which
triggers the addition of the ":BOOT_TIME" part to the file name).

> >>| commit 393f58c85aeb78f814866ccaad9ae7efd3fa6766
> >>| Author: Adam Porter <adam@alphapapa.net>
> >>| Date:   Fri Mar 8 23:43:14 2024 -0600
> 
> > This is extremely unlikely to have caused the problem.  It is more
> > likely that some change in Cygwin and/or in the Gnulib's boot-time
> > module causes this now.
> 
> Indeed.  But the fact is that Emacs built with that revno adds
> a colon and a negative number to a link name and the one built
> with just before it does not.  No problem on 29.3.50 and earlier,
> either (i.e., not add a colon and numbers).

Then please step with a debugger through the relevant code in
filelock.c, before and after that change, and see why they are
different.  I cannot explain that, even after doing "git log -L" on
all of filelock.c: the most recent change of the related code was in
Aug 2023, AFAICT, which is a very far cry from Mar 2024.

And even if we understand why you now get the ":BOOT_TIME" part,
there's still the question why it cannot be unlinked, if Cygwin
supports file names with a colon.  Maybe it's because Emacs creates a
symlink, and symlinks with such names are not supported by Cygwin?